We report results of beam tests of the FONT3 intratrain
position feedback system prototype at the
Accelerator Test Facility (ATF) at KEK. The feedback
system incorporates a novel beam position monitor
(BPM) processor with latency below 5 nanoseconds, and
a kicker driver amplifier with similar low latency. The 56
nanosecond-long bunchtrain in the ATF extraction line
was used to test the prototype feedback system. The
achieved latency of 23ns provides a demonstration of
intra-train feedback on very short timescales relevant
even for the CLIC Linear Collider design.